The present invention relates generally to a socket for mounting a tube having a plurality of conductive contact elements disposed about the neck thereof, and the method of using the same, and more particularly to sockets for mounting such tubes on printed circuit boards and affording spark gap protection to critical circuits.
Cathode ray tubes are presently in wide use in the electronics industry. Although they perform a variety of functions, these tubes are probably most well known as picture tubes in television receivers. Conventional cathode ray tubes generally have a plurality of pins or leads extending in a ring-like configuration from the base of the tube to enable the tube pins to be plugged into the "enveloping" pin terminal contacts of the socket and thus become connected to the remainder of the electrical components. The normal operating potential of all the pins, except one, is in the range of a few thousand volts. However, the remaining pin, often referred to as the high voltage pin, operates at a substantially high voltage, usually in the neighborhood of 25,000 volts DC or greater.
In order to prevent damage to the cathode ray tube caused by excessive voltage at the pins, cathode ray tube sockets are normally provided with some sort of safety device in the form of grounding apparatus. Usually the grounding apparatus permits a spark to jump from the pin contact to ground in the event that the pin is operating at an excessive potential. This prevents damage to the tube due to excessive potential at one of the pins by providing a non-destructive path to eliminate the excess.
In the past, the grounding apparatus has commonly taken the form of a single grounding plate which is spaced from the pin terminals in the socket by means of an insulating plate. The insulating plate or spacer is provided with holes aligned with the pin terminals to permit the spark to jump from the terminal to the grounding plate. The distance between the grounding plate and the terminal will determine the potential necessary to permit a spark to bridge the gap between the terminal and the grounding plate. Sockets offering spark gap protection of the type described are the subject matter of Pittman U.S. Pat. No. 3,423,720 (issued Jan. 21, 1969) and Offerman U.S. Pat. No. 3,728,587 (issued Apr. 17, 1973), both assigned to the assignee of the present application.
Because of the normal operating range of the high voltage pin, the space between the high voltage pin terminal and the grounding plate is usually greater than the space between the remaining pin terminals and the grounding plate so that a higher potential is necessary before a spark can bridge the gap between the high voltage pin terminal and the grounding plate. However, this grounding configuration has a very great disadvantage. Specifically, if a spark jumps from the high voltage pin terminal to the ground plate, the potential of the ground plate may rise faster than it can be dissipated to the ground. A second spark may then jump from the ground plate back to one of the other pin terminals, thus seriously damaging the tube. To overcome this disadvantage, cathode ray tube sockets have been provided with a second grounding plate for grounding the high voltage pin, the second grounding plate being physically and electrically independent of the low voltage grounding plate. The high voltage grounding plate has a separate grounding wire extending therefrom to an appropriate grounding member, such as the chassis of the electrical component, and is physically and electrically separated from the low voltage grounding plate. In this way, a spark which has jumped from the high voltage terminal to the high voltage grounding plate cannot jump back to any of the remaining contacts to damage the tube. Sockets describing such a dual spark gap protection grounding system are described in Pittman U.S. Pat. No. 3,865,452 (issued Feb. 11, 1975), also assigned to the assignee of the present invention.
In all of the above-described systems, the sockets utilize discrete wire leads connected to the individual pin contacts of the sockets which engage the contact elements extending from the base of the cathode ray tube. More recently, such sockets have been adapted to eliminate, for the most part, the discrete wire lead system by mounting the socket directly to a printed circuit board. Because of the limited potential handling capability of the printed circuit board, the high voltage lead and, in some cases, the ground lead are maintained as discrete wire leads. The contact members of the sockets (replacing the discrete wire leads) are passed through small corresponding apertures of the printed circuit board and then soldered to the exposed side of the printed circuit board. Typically the soldering operation utilized to secure the socket to its printed circuit board mounting is either a dip solder or wave solder operation. In the dip solder operation, the portions to be soldered (the exposed printed circuit board surface and the contact ends projecting downwardly therefrom) are actually immersed in a molten solder bath. In the wave solder operation, the surfaces to be soldered are spaced slightly above a molten solder bath, and a wave is created in the molten solder bath which is of sufficient height to effectively wet the portions to be soldered. As the enveloping pin terminal contacts of the socket are disposed on the surface of the socket opposite the surface being exposed to the solder and are typically spaced from any central aperture of the socket housing, judicious selection of the immersion depth in the dip solder operation or of the wave height in the wave solder operation suffices to insure that solder does not enter the enveloping pin terminal contacts and accidentally form a short circuit between adjacent contacts.
In response to the recent introduction of a new type of cathode ray tube (commonly called a "small neck CRT"), it has been desirable to modify the above-described sockets. In the small neck CRT, the contact elements do not extend from the base of the tube, but rather are disposed along the periphery of the neck of the tube, adjacent to the base. This requires the use in the socket of what is commonly referred to as a "single sided contact" rather than the enveloping type of pin terminal contact heretofore used in such sockets. The single sided contacts are disposed about the periphery of (and typically at least partially within) a large central aperture of the socket, the small neck CRT being mounted on the socket with the neck portion thereof adjacent the base being disposed within the large central aperture of the socket. The modified socket is designed to be mounted directly on a printed circuit board, with discrete wires being used only for the high voltage and ground connections, the remainder of the contacts being soldered to the exposed surface of the printed circuit board as in the past. However, the printed circuit board is provided with a large central aperture therethrough aligned with the large central aperture of the socket, so that the tube neck may be inserted into the socket through the printed circuit board. As a result of this design, problems have been encountered during the soldering operation as the solder is sucked up through the large center aperture of the printed circuit board and into the large center aperture of the socket due to a chimney effect of the aligned large central apertures. This is obviously undesirable as the solder entering into the large central aperture of the socket is capable of forming short circuits between the spaced single sided contacts of the socket extending into the large central aperture, thus requiring rejection of the socket.
In order to overcome this disadvantage of the modified socket design, a removable cardboard plug is conventionally inserted into the large central aperture of the socket intermediate the single sided contacts and the surface of the socket adjacent the printed circuit board. The cardboard disc is pressed into the large central aperture of the socket just enough to guarantee its integrity and retention by the socket. After the soldering operation has been completed, the cardboard disc is then pushed out of the large central aperture of the socket and through the large central aperture of the printed circuit board, thus exposing both of the large central apertures for receipt of the tube neck. While the use of the cardboard disc has provided a solution to the problem raised by the modified socket design, the need to provide the cardboard disc and to insert it into the socket has added to the manufacturing cost of such removably plugged sockets. Furthermore, during the process of inserting the cardboard disc into the socket, it is possible for the cardboard disc to become slightly bent out of shape, thereby creating a gap (between the cardboard disc and the adjacent socket wall defining the large central aperture) through which the solder can pass.
Accordingly, it is an object of the present invention to provide a socket for mounting a tube having conductive contact elements disposed along the neck thereof to a printed circuit board which includes a removable solder plug for protecting the single sided contacts of the socket at essentially no additional material or processing costs.
Another object is to provide such a socket which precludes the possibility of gaps between the removable solder plug and the socket wall defining the large central aperture.
Still another object is to provide a socket in which the removable solder plug is formed of the same material as the insulating plate or spacer and is created during and as part of the creation of the insulating plate from material heretofore discarded after creation of the insulating plate.
A further object is to provide a method of mounting such a socket on a printed circuit board without the step of first inserting a removable solder plug into the central aperture of the socket.